Acrux is brightest star in Southern Cross

Blue Acrux shines as the brightest star in the constellation Crux the Southern Cross.

Bluish Acrux, otherwise known as Alpha Crucis, is the brightest star in the constellation Crux the Southern Cross. It is the 13th brightest star in all the sky. Because it is located far to the south of the sky’s equator, this star cannot be seen from much of North America. Observers north of central Florida or south Texas are out of luck. Observers in Hawaii sometimes tell us they’ve spotted it. You have to be in the Southern Hemisphere to see Crux in all its glory. Therein lies much of its magic and mystery. Follow the links below to learn more:

How to see Acrux. For anyone south of about 27 degrees north latitude, Acrux and the Southern Cross exhibit their midnight culmination (highest elevation above the southern horizon) in late March amd early April. The farther south the better, and from approximately the latitude of Brisbane, Australia, the star becomes circumpolar and can be seen every night of the year.

The star Acrux marks the bottom of the Southern Cross, when you are facing south. For an observer facing south, Acrux is the star nearest the horizon. At magnitude 0.77, Acrux is the most southernly first magnitude star. Nearby Mimosa, or Beta Crucis, at magnitude 1.25, is the 19th brightest star in all the heavens.

Two nearby and brighter stars, Alpha Centauri and Beta Centauri (Hadar), are known as the Southern Pointers. A line drawn from Alpha through Beta, at about 3 times the distance between them, leads to the top of the Southern Cross, Theta Crucis.

Samuel Langhorne Clemens (1835 – 1910), better known as Mark Twain, in the year 1902. Although Twain was unimpressed with the Southern Cross and its brightest star Acrux, many have thrilled to seeing this constellation in southern hemisphere skies. Image via the State Historical Society of Missouri

History and mythology of Acrux. Unlike many star names, which are Arabic, Latin or Greek proper names, Acrux is simply a combination of “A” (for Alpha) and Crux as the name of the constellation. Not of classical derivation at all, it was a name coined by Elijah Hinsdale Burritt, a Connecticut farm boy turned celestial cartographer, circa 1835.

While little if any mythology is known to be associated with this star, and not much more with the entire constellation, the reputation of the Southern Cross is widespread. That might be because early sailors, moving southward on Earth’s globe, thrilled to the sight of it.

The Northern Cross, an asterism of the main stars of Cygnus, the Swan, is larger and better shaped as a Christian Cross, although it contains no stars as bright as Acrux. Interestingly, however, the brightest star in Cygnus, Deneb, is virtually the same apparent magnitude as Mimosa, the second brightest star in Crux. Both are bright blue stars of magnitude 1.25, ranking as the 19th (Mimosa) and 20th (Deneb) brightest stars in the sky.

Many point out that Crux, of which Acrux is the primary star, does not look exactly like a cross. They say it is as exaggerated as were the rumors of Mark Twain’s death well before his actual demise. In fact, Twain wrote of the Southern Cross, viewed for the first time and reported in his book, Following the Equator:

We saw the Cross to-night, and it is not large. Not large, and not strikingly bright. But it was low down toward the horizon, and it may improve when it gets up higher in the sky. It is ingeniously named, for it looks just as a cross would look if it looked like something else. But that description does not describe; it is too vague, too general, too indefinite. It does after a fashion suggest a cross — a cross that is out of repair – or out of drawing; not correctly shaped. It is long, with a short cross-bar, and the cross-bar is canted out of the straight line.

It consists of four large stars and one little one. The little one is out of line and further damages the shape. It should have been placed at the intersection of the stem and the cross-bar. If you do not draw an imaginary line from star to star it does not suggest a cross – nor anything in particular.

One must ignore the little star, and leave it out of the combination – it confuses everything. If you leave it out, then you can make out of the four stars a sort of cross – out of true; or a sort of kite – out of true; or a sort of coffin-out of true.

Despite Twain’s reluctance to be impressed, many love the Southern Cross, and today this constellation is used as a symbol on the flags of several nations in the southern hemisphere.

Image Credit: Wikimedia Commons

The star Acrux, aka Alpha Crucis, appears single to the eye. But a telescope reveals it to be two stars orbiting around each other. Image via Starfield Observatory

Acrux science. Using data from the Hipparcos mission. Acrux is classified as B0.5 IV. meaning that it is hotter, brighter, larger and more massive than our sun. The designation “IV” indicates that Acrux is a “sub giant” star, not big enough to be considered a giant, but one that has left the realm of normal stars (the “main sequence”) and has entered the terminal phases which ultimately will end up as a white dwarf star.

Acrux is about 321 light-years from Earth, and in fact it is not one star but two nearly identical B-class stars. These can be resolved in a small telescope, making Acrux a nice double, but they appear as a single star to the unaided human eye. The brighter of the two — call it Alpha-1, is magnitude 1.33 and the dimmer — call it Alpha-2 –1.73, with a resultant magnitude of 0.77. Alpha-1 is a subgiant, whereas Alpha-2 appears to still be on the main sequence, rather oddly termed a “dwarf” star. (All stars on the main sequence are technically considered “dwarf stars” to distinguish them from subgiants and giants — there are no “normal” stars!) According to Dr. James Kaler, the subgiant boasts a temperature of about 25,000 Kelvin at its surface, whereas the smaller star is also cooler, about 16,000 Kelvin.

To make things even more complicated, it appears that Alpha-1 is in fact a double star, whose components’ combined masses are about 24 times that of the sun. Alpha-2 is a single star about 13 times as massive as the sun.